How to Make the Best Slow Cooker Caramelized Onions

It takes time and patience to transform eye-watering, nose-assaulting, sharp-tasting raw onions into soft and nutty-sweet caramelized onions. Done stove-top, it’s a labor-intensive, 30- to 45-minute cook time that can’t be hurried along if you want the food science to work. But the good news is, there's an easier way to make caramelized onions: Let your slow cooker do all the work.

Photo by Vanessa Greaves

Low and slow from the get-go, a slow cooker eliminates the frequent stirring and constant tending needed to ensure the onions don’t go from brown to burnt behind your back. Bonus: You can make a large quantity to bank for multiple recipes, saving you loads of prep time down the road.

What to Know before You Get Started

Any onion can be caramelized. You don’t have to spend more for sweet onions like Vidalias or Walla Wallas. Inexpensive yellow onions caramelize beautifully.

Photo by Vanessa

Put down that sugar bowl. Despite what some recipes may tell you, you do not have to add sugar to make caramelized onions. Cooking the onions for a long time brings out the natural sugars in the onions, turning them soft, brown, and super-sweet. It's a complicated science and not entirely understood, but it works. Trust.

It takes a lot to make a little. I found that four quarts of sliced raw onion cook down to about 1 quart of caramelized onion. Of course, your yield may vary.

Photo by Vanessa Greaves

Your home will smell like onions. Be warned. That big pile of sliced raw onions will grab you by the nose for a while until it cooks down a bit. Then it starts to smell amazing. In the meantime, open a window, turn on a fan, or use the slow cooker method overnight or while you're not home.

Allow lots of time. Slow cooking for 10 hours on LOW gives you golden brown onions with a lot of liquid to drain off. (Save the liquid to flavor soups, stews, rice, and pasta dishes.) You can stop at that point, or leave the liquid in and cook for an additional 4 or 5 hours on LOW with the lid ajar. This cooks off most of the extra liquid and gives you even sweeter, deeper brown onions -- almost like onion jam.

Photo by Vanessa Greaves

Storing Caramelized Onions

If you’re going to use up all the onions within a week, simply cool and refrigerate in an airtight container. To freeze for later, divide into smaller portions to freeze in ice cube trays and muffin tins, then transfer to plastic bags. The quality should be good for up to 3 months.

Directions

Sprinkle onions with the melted butter or olive oil and salt. Toss to coat.

Cook for 10 hours on LOW. At this point, I found the onions had reduced from about 4 quarts raw to a little over 1 quart cooked onions and 1 1/4 cup liquid. To reduce further, leave the liquid in and cook for another 4 or 5 hours on LOW, stirring occasionally, until the onions are deeper brown and the liquid is almost completely cooked off.

*Note: I used a 6 1/2-quart slow cooker. Adjust the recipe amounts to suit the size of your equipment.

Using Caramelized Onions

Now that you've got a stash of caramelized onions cooked up and ready to go, you can make recipes like French onion soup in a fraction of the time it normally takes.